A lock-up clutch is a clutch that directly couples an input side and an output side of a torque converter. When a vehicle operating condition, defined on the basis of a vehicle speed and a throttle opening, is in a predetermined region, a slip amount of the lock-up clutch is slip-controlled to a target slip amount.
If an engine torque varies rapidly during the slip control, tip-in shock occurs. JP06-174075A discloses a technique according to which a target slip amount is increased when a variation rate of the throttle opening equals or exceeds a predetermined value during the slip control, and when the variation rate of the throttle opening falls to or below a predetermined value thereafter, the target slip amount is reduced gradually over a time period corresponding to the increase in the variation rate of the throttle opening.